1. Field of the Invention
This invention relates to a heavy duty pneumatic tire. In particular it relates to a heavy duty pneumatic tire effectively eliminating railway wear often occurring in such a tire used for continuous travelling at high speeds for long distances.
2. Description of the Prior Art
In general, radial tires using metal cords for reinforcing belts have various advantages such as high wear-resistant property, punctureproof and the like because of the stiff belts arranged between tread rubbers and carcass plies different from usual bias tires. However, radial tires are uncomfortable in riding because of the reinforcing effect resulting from the stiff belts. Therefore, although the radial tires may be used on bad roads, they have been developed for good roads and have been remarkably widely used with the considerable improvement of road conditions such as the development and accomplishment of superhighways.
For these purposes, treads of these tires are in general formed with zigzag ribs extending in circumferential directions of the tires in consideration of the traction and braking performances and wear-resistant property and for preventing heating. Such tread patterns are generally referred to as "rib-type" pattern.
Such ribs are usually continuous in the circumferential directions of the tires but are discontinuous in the circumferential directions by traverse grooves in directions of tire widths interrupting the ribs as the case may be. In many cases, extraordinary wear (referred to as "irregular" wear hereinafter) tends to occur in the treads having such ribs in continuously straight running at high speeds for long distances.
As can be seen in more detail in FIG. 1, the irregular wear partially occurs in areas denoted by R in FIG. 1 from a first used stage to an intermediate stage. The irregular wear starts at corners 3, particularly point ends 3' of the circumferential ribs 2 extending in width directions of the tire into the zigzag tread grooves 1 extending in the circumferential directions of the tire and then expands along side edges 2' of the ribs 2 in the circumferential and width directions of the tire. The irregular wear then increases its width w with a depth .differential. as viewed in a section shown in FIG. 2 during the intermediate stage and thereafter. The worn areas R then progressively increase with the running distance of the tire beyond retracting corners 4 of the ribs 2 between the extending corners 3 so as to be joined with each other along side edges 2' of the circumferential ribs 2 to form continuous worn areas in the tread of the tire T. The depth .differential. and width w of the wear then further increase with running distances of the tire. Such an irregular wear is generally referred to as "railway" wear. The depressions at the edges of the tread grooves due to the railway wear make the tire unaesthetical and adversely affect the tire performance depending upon functions of the edges of the tread grooves 1, particularly the traction and braking performances to greatly reduce the life span.
It has been generally known that the railway wear is usually caused by stress concentrations in traverse directions in the proximity of the point ends 3' of the extending corners 3 of the circumferential ribs 2 in running at high speed. In view of this, various solutions have been proposed for overcoming the above disadvantages.
One solution is to provide a plurality of narrow cuts with circumferential intervals at least in side edges of circumferential ribs defined by circumferential grooves in a central zone of a tire tread as disclosed in U.S. Pat. Nos. 3,550,665 and 3,954,130. With such cuts having, for example, substantially the same length in the circumferential ribs, however, the cuts in the proximity of retracting corners of the ribs are longer than those required for this purpose, so that the rigidity of the circumferential ribs as a whole is reduced to shorten the life due to wear, or the cuts in the extending corners of the ribs in which the railway wear would start are insufficient in length to prevent the railway wear. Accordingly, such cuts are required to have suitable lengths, depths and so on.